An organic light emitting display (OLED) is one of hot topics in the field of flat panel display. Compared with a liquid crystal display, OLED has advantages of low power consumption, low production cost, self-luminescent, broad view angle and fast response speed and so on. At present, in the display field such as mobile phone, PDA and digital camera and the like, OLED has taken the place of the traditional LCD display screen. Unlike that LCD controls luminance by using a stable voltage, OLED is current-driven and a stable current is required to control light emitting. Due to manufacturing processes, device aging and so on, non-uniformity exists in a threshold voltage Vth of a driving transistor of a pixel circuit, thereby resulting in that a change occurs in the current flowing through OLED of each pixel point such that display luminance is non-uniform, which influences display effect of the entire image.